Add a HW op for getting spare blocks.

12xx cards require 2 spare blocks for GEM encrypted SKBs, regardless
of VIFs or keys programmed into the FW.

18xx cards require 2 spare blocks when there are any connected TKIP or
GEM VIFs. For now always return 2 spare blocks, as this works with all
networks. The special case TKIP/GEM functionality is added at a later
patch.
Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>